(1) Field of the Invention
The present invention relates to a two-engine power plant having an emergency system for injecting fluid, and it also relates to an aircraft.
More particularly, the invention relates to a power plant of a rotorcraft and in particular of a helicopter. The technical field of the invention is thus the technical field of power plants, in particular for rotorcraft type aircraft.
(2) Description of Related Art
A rotorcraft has a power plant for driving its rotary wing in rotation. The power plant has one or more engines for driving a main gearbox of a helicopter, this main gearbox then driving the rotary wing in rotation.
It should also be understood that throughout the present specification, the term “engine” covers not only turboshaft engines but also piston engines, for example.
Each engine is capable of operating at a plurality of operating ratings.
For example, thermal limitations on an engine and torque limitations on the main gearbox serve to define three normal operating ratings for the engine of a rotorcraft:
takeoff rating, corresponding to use that damages neither the main gearbox nor the engine during a takeoff of limited duration, which duration usually lies in the range 5 minutes (min) to 30 min, with this being referred to as takeoff power (PMD);
a maximum continuous rating corresponding to use that damages neither the main gearbox nor the engine over a duration that is not limited: this is known as maximum continuous power (PMC); and
maximum transient rating, optionally having limits set by regulation: this is referred to as maximum transient power (PMT).
There also exist supercontingency ratings for multi-engine rotorcraft that are used in the event of an engine failing:
a first contingency rating during which the mechanical potential of the inlet stages of the main gearbox and the temperature potential of the engine are used to the maximum: this rating can be used for a maximum of 30 seconds consecutively and on at least three occasions in a flight, it is referred to as the PSU rating, and if is used that can require the engine to be removed for overhaul;
a second contingency rating during which the potential of the inlet stages of the main gearbox and the potential of the engine are used to a great extent: this rating may be used for two minutes after using the PSU rating or for two minutes thirty seconds consecutively, at most, and it is referred to as the PMU rating; and
a third contingency rating during which the potential of the inlet stage of the main gearbox and the potential of the engine are used, but without being damaged: this rating may be used for thirty minutes or continuously for the remainder of the flight after the failure of an engine and is referred to as the PIU rating.
Nevertheless, the power developed by an engine during a given rating may be barely sufficient under particular conditions, e.g. in a hot atmosphere.
It is then known to inject a fluid into an engine in order to increase its power. The fluid may in particular be pure water or a mixture of water and alcohol. For example, in an engine having a free turbine, it is possible to inject a water-based fluid in order to increase the power developed by the engine without increasing the temperature of the gas at the outlet from the combustion chamber of that engine.
The concept of injecting pure water was used on piston engines during the second world war and has since been used on airplanes with turbojets.
The water may be injected into the combustion chamber of the engine. Consequently, the composition of the gas generated by the gas generator changes. This leads to a modification to the heat capacity and to the bulk enthalpy of the gas. For constant flow rate of gas created in the combustion chamber, the power generated by the engine increases with increasing bulk enthalpy of the gas.
When the engine is a turboshaft engine having a gas generator with an air inlet and a compressor upstream from the combustion chamber, the water may be injected into the air inlet.
Under such circumstances, the total mass flow rate through the air inlet increases, thereby delivering extra power at constant bulk enthalpy. The power available at constant combustion temperature and at constant air flow rate is greater.
Nevertheless, the fuel flow rate is also increased at constant combustion temperature and at constant air flow rate.
A fluid may be injected in the form of a mist.
Consequently, an emergency fluid injection system can be used on a multi-engine aircraft in the event of an engine failing in order to increase the power of the engines still in operation. Nevertheless, such a system can be difficult to develop while also satisfying severe safety targets. Such an emergency system can be difficult to test.
Thus, an emergency system may comprise a pump for conveying a water-based fluid to an engine in order to increase its power. Nevertheless, safety requirements may then impose providing the pump in redundant manner, thereby leading to a system that is relatively heavy and/or bulky.
Document FR 2 859 761 describes a system for providing protection against over-stress in a turbine engine. That system includes an injector device for injecting a cooling agent into the engine when the outlet temperature of the gas exceeds a safe temperature.
The injector device comprises a tank connected to an injector by a duct. A pump and a valve are also arranged along the duct.
Document U.S. Pat. No. 5,784,875 indicates that the combustion of fuel in a turbine engine creates pollution: specifically nitrogen oxides known as NOx.
That Document U.S. Pat. No. 5,784,875 describes a device for reducing NOx emission by injecting water into the combustion chamber. The device comprises both a pipe for conveying air taken from the engine and provided with a first valve, and also a pipe for conveying water that is provided with a second valve. The device then opens the first and second valves in order to inject into the combustion chamber a mixture containing the air taken from the engine, water, and fuel.
Document FR 2 826 094 discloses a system for lubricating and cooling a mechanical assembly that includes an emergency lubrication device that is put into operation automatically or manually when the main device fails. The independent emergency device has at least one tank of lubricant and cooling liquid; and at least one source of gas under low pressure; at least one spray nozzle that is fed firstly with lubricant and cooling liquid under pressure from the tank and secondly with gas under pressure from said source and that is suitable for spraying a mist of lubricant and cooling liquid onto the mechanical assembly in order to take over temporarily from the failed main device.
Document EP 2 333 247 relates to a method of increasing the safety of a power plant having at least one engine and a main gearbox, the engine driving the main gearbox. That main gearbox includes a lubrication system provided with the help of an aqueous medium stored in a tank. Under such circumstances, according to that method, a fluid comprising water is injected into the engine in order to increase the power developed by the engine without increasing the temperature of any member of the engine, or in order to reduce that temperature without modifying the power developed by the engine, the fluid being taken from said tank.
Also known are Documents U.S. Pat. Nos. 3,434,281, 3,518,023, and GB 2 079 707.